When X7R SMD capacitors are used within a over current protection circuit of a half bridge converter (supply voltage 400Vdc) a strange phenomenen occurs that the X7R caps show a high non linear behaviour. The over current protection circuit becomes highly sensitve for load steps. The OCP trips as the capacitors seem to have a higher value at first large dv/dt step. Sometimes the effect is less or even absent. When NP0 (or COG) capacitors are used this behaviour is completly absent. It would be interesting to know why the X7R capacitors are so sensitive for high voltage/high dv/dt signals.
This is due to electrostrictive effect of dielectric materials. When an AC voltage is applied, the capacitor effectively stretches and shrinks in the different dimensions. Mechanical distortion and stress concentrate around the edge of the external electrode of the capacitor. If the AC voltage is large, and the material has sufficient dielectric properties, after some time cracks can form under the external electrode which can lead to short circuit of the component.
There are ways to mitigate this effect as outlined below:
First factor to take into account is the thickness of the dielectric material between the inner electrodes of the capacitor. This has a direct effect on the magniture of the electrostrictive phenomenon. When constructing high voltage capacitors, it's therefore essential to ensure the dielectric layer is thick enough to reduce the chip capacitor's internal mechanical stress.
Secondly, the structure of the inner electrodes can contribute to the electrostrictive effect.
Additionally, thickness of the outer layer of the dielectric (the dummy layer) can influence the formation of cracks.
Last but not least, properties of the dielectric material itself can play a part. Murata carefully consider tjhe properties of its materials to ensure that product specified for high voltage capacitor are constructed from suitable dielectric materials.
Thanks a lot for this useful information. The design which failed with the X7R capacitors is not showing any strange behaviour when fitted with NP0. The quality is consistent for the last year's production. This shows again the importance of carefully selection of components (in the development process as well as in the production process).
Thanks a lot for this useful information. The design which failed with the X7R capacitors is not showing any strange behaviour when fitted with NP0. The quality is consistent for the last year's production. This shows again the importance of carefully selection of components (in the development process as well as in the production process).
